1. Field of the Invention
The present invention relates to an FM (frequency) modulation) demodulating device which ensures to robustly demodulate a received FM signal even under a jammed communication condition through the use of an improved extended Kalman filtering scheme which is applied to the FM communication system exhibiting random occurrences of a cochannel.
2. Description of the Conventional Art
In general, a communication system may receive the desired significant signal, linearly with unwanted interfering and noise signals. In particular, a stronger power of the interfering signal relative of the information signal makes it difficult to communicate with much satisfaction. Under a military-purpose communication, a jammer is often used in transmitting signals of frequencies selected at random so as to disturb radio channels of an opponent and thus the communications thereof. In addition, a cochannel interference founded in a commercial or military communication randomly occurs, which acts as a cause of a disturbance of communication.
To this end, a conventional communication system employs two phase-locked loops to overcome those problems, whose purposes are to track the desired and interfering signals, respectively.
The signals modeled as represented in the following equation (1) are applied to an extended Kalman filter (EKM) from which the states of the phase, frequency and amplitude are estimated: EQU Y(t)=S.sub.d (t)+S.sub.i (t)+n(t) (1)
where Y(t) denotes observations, S.sub.d (t) the desired signal, n(t) noise, and S.sub.i (t) interfering signals, respectively.
There has been a problem in that due to the random occurrence of the cochannel phenomenon the communication in a better condition cannot be attained, resulting in the quality of the communication being greatly reduced.